Local geomorphic effects in the presence of accumulations of different densities

Mobile components of dynamic river systems are varied, and more than fundamental water and sediment interactions must be considered to assess morphodynamic changes. Here, the effects of instream wood and plastic accumulations on the riverbed are modelled and assessed in the presence of a hydraulic structure. We observe a recent tendency towards considering instream wood and plastics as integral riverine system components. Such shift is driven by a multidisciplinary interest in instream wood and riverine plastic dynamics, including those addressing flood hazards and risk assessments. Like sediment, instream wood and plastics can be transported along the river channels during floods, increasing flooding risks and hazards by accumulating near hydraulic structures. Collection racks, a commonly encountered hydraulic structure, are used for trapping and removing various accumulated material, mitigating any potential risks. However, accumulations at collection racks can cause other risks because of potential backwater rise and local scour development. Accumulations of different densities, such as a combined presence of instream wood and plastics, add further complexity to the process understanding. In this study, a physical model is used to explore the geomorphic effects of accumulations of two different densities at a collection rack. Local geomorphic changes (i.e., scour and deposition), their evolution and the spatial extent around a vertical collection rack were analysed with Structure from Motion photogrammetry. The results show a strong relationship between the accumulation density and the hydraulic and geomorphic changes. Light floating material, forming accumulations closer to the water surface, are associated with the largest geomorphic changes, in contrast to dense material, forming accumulations close to the riverbed. In the presence of different densities, intermediate geomorphic changes were observed. Finally, to discuss the observations, we present a conceptual flow field for different scenarios of accumulation densities. Ultimately, outcomes from this work are essential to improve design practices for river managers.